2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 10
Presentation Time: 8:00 AM-12:00 PM

WHOLE-ROCK ND (ID-TIMS) AND ZIRCON HF-U-PB (LAMS) ISOTOPE COMPOSITION OF MESOPROTEROZOIC A-TYPE GRANITES IN MANNEFALLKNAUSANE, WESTERN DRONNING MAUD LAND, ANTARCTICA


RÄMÖ, O. Tapani1, LUTTINEN, Arto V.1, ANDERSEN, Tom2 and KURHILA, Matti I.1, (1)Department of Geology, Univ of Helsinki, P.O. Box 64, Kumpula, Physicum, Helsinki, FIN-00014, Finland, (2)Department of Geosciences, University of Oslo, P.O. Box 1047, Oslo, N-0316, Norway, tapani.ramo@helsinki.fi

Crystalline bedrock in western Dronning Maud Land, Antarctica is composed of the Archean Grunehogna craton and the Mesoproterozoic Maud mobile belt. ~100 km south of Archean-Proterozoic transition, in the Proterozoic realm, nunataks of Mannefallknausane (~74.5oS, 15oW) are dominated by Precambrian granitoid rocks and rare paragneisses. Three principal granites can be identified: a white, garnet-bearing K-feldspar-megacrystic biotite granite; a red biotite-hornblende±clinopyroxene granite with and without plagioclase-mantled K-feldspar-megacrysts (rapakivi texture); and a dark green porphyritic charnockite with orthopyroxene and hornblende. For two varieties of the red granite (wiborgite, pyterlite), our new U-Pb data imply crystallization ages of 1073 ± 6 Ma and 1084 ± 8 Ma, respectively. These are compatible with a U-Pb zircon upper intercept age of 1073 ± 8 Ma of the charnockite (Arndt et al., 1991). The initial Nd isotope composition of these rocks is relatively radiogenic [εNd (1075 Ma) value of the biotite granite -0.5; red granite +0.3, +0.5; charnockite +1.4], as is that of a country-rock gneiss from the surrounding bedrock (+1.0). Initial zircon εHf values of the wiborgite and pyterlite are +5.2 ± 2.2 (2σ) and +6.3 ± 1.8 (2σ), respectively, and thus confirm the juvenile character of the red granite. The εNd (1075 Ma) value of a Proterozoic mafic plutonic rock from the nearby Heimefrontfjella region is clearly more positive (+4.1) than those of the A-type association of Mannefallknausane, implying a depleted mantle source. The A-type granite-charnockite association in western Dronning Maud Land was emplaced into a comparatively juvenile crust at 1075 Ma and has Nd and Hf isotope systematics that are compatible with a major Mesoproterozoic crustal source component.